1. Field of the Invention
This invention relates to a radiation detector adapted for use in a radiation imaging apparatus, such as an X-ray imaging apparatus.
2. Description of the Related Art
With respect to radiation imaging operations for medical diagnoses, there have heretofore been known radiation imaging apparatuses, in which radiation image detectors (utilizing semiconductors as principal sections) are utilized for detecting radiation and converting the radiation into an electric signal. Ordinarily, the radiation image detectors may be classified into a direct conversion type, wherein the radiation is directly converted into electric charges, and wherein the thus formed electric charges are accumulated; and an indirect conversion type, wherein the radiation is converted into light by use of a scintillator, such as CsI:Tl or GOS (Gd2O2S:Tb), wherein the thus obtained light is then converted into electric charges by use of a photo-conductor layer, and wherein the thus formed electric charges are accumulated. Also, in accordance with read-out techniques, the radiation image detectors may be classified into an optical read-out technique, wherein the electric charges having been generated with the irradiation of the radiation are accumulated at a charge accumulating section, and wherein the accumulated electric charges are read out by the utilization of a semiconductor material capable of generating the electric charges when being exposed to light; and an electric read-out technique, wherein the electric charges having been generated with the irradiation of the radiation are accumulated at accumulating capacitors, and wherein the accumulated electric charges are read out through an operation, in which an electric switch, such as a thin film transistor (TFT), a charge coupled device (CCD), or a complementary metal oxide semiconductor (CMOS) sensor, is turned on and off with respect to each of pixels.
The direct conversion types of the radiation image detectors are constituted for performing a radiation detecting operation, wherein a predetermined bias voltage is applied across a voltage applying electrode, which has been formed on a surface of a radiation-sensitive photo-conductor film, and a charge collecting electrode, which has been formed on a substrate, wherein the electric charges having been generated with the irradiation of the radiation are collected by the charge collecting electrode, which been formed on the opposite surface of the photo-conductor film, and wherein the thus collected electric charges are taken out as a radiation detection signal. By the application of a high voltage to the voltage applying electrode located on the photo-conductor film, the electric charges having been generated by the radiation are collected efficiently and are taken as a charge latent image. In the cases of the direct conversion types of the radiation image detectors, wherein the high voltage is applied to the voltage applying electrode, an edge region of the voltage applying electrode is set at a particularly high electric field, and therefore the problems are encountered in that breakdown from the edge region of the voltage applying electrode is apt to occur.
In view of the problems described above, there has heretofore been employed a technique, wherein a thick electrical insulator is located under the edge region of the voltage applying electrode, and wherein the breakdown at the edge region of the voltage applying electrode is thereby suppressed. (Reference may be made to, for example, U.S. Pat. No. 6,885,005.)
However, in cases where the electrical insulator is located as described above, new problems occur in that the electric charges which should be recombined at the voltage applying electrode remain without being recombined. Particularly, it has been found that, in cases where the radiation detector is used repeatedly, the problems occur in that the breakdown becomes apt to occur even further due to the electric charges remaining without being recombined owing to the electrical insulator, which has originally been provided for suppressing the breakdown.